The invention relates to a distance measuring device for use in cameras or the like.
Various types of distance measuring devices of the light projecting and receiving type have been proposed which incorporate integrating circuits. Such devices operate by activating a light projecting circuit to emit light for a predetermined number of times or for a predetermined period of time onto an object, integrate a received light reflected by the object and perform A/D conversion of the integrated voltage to calculate a distance to the object undergoing distance measurement. Such distance measuring devices require several comparators as shown, for example, in Japanese Patent Laid-Open No. 119307/1991, to convert interterminal voltages of integrating capacitors into digital signals. In fact, the resolution of such devices is proportional to the number of comparators. Thus, to attain higher accuracy distance measurement, the distance measurement circuit becomes complex and large-scaled, which results in a high cost.
To solve foregoing problem, the assignee of the present application has proposed in Japanese Patent application No. 291342/1992 a distance measuring device in which a pulsed light is irradiated onto an object undergoing distance measurement, light reflected by the object is integrated by two light receiving circuits for long and short distance ranges of the entire measurable distance range until the integrated voltage reaches a predetermined value, the number of light projections at that time is counted, and the distance to the object is calculated on the basis of two count values thus obtained by the respective light receiving circuits. However, the above-described distance measuring device does not take into account the influence of drift voltages of internal amplifiers due to temperature change. Accordingly, the drift voltages are superimposed on outputs of integrating circuits when the ambient temperature varies, which may cause degradation in distance measuring accuracy. While such drift voltages can be removed by the addition of circuitry, this results in increasing the size and complexity of the circuit and makes the distance measurement device expensive due to the addition of a special purpose circuit for the removal of drift voltages.
To solve the above-mentioned problem, the assignee of the present application has proposed in Japanese Patent application No. 320369/1992 a distance measuring device in which the accuracy of distance measurement is enhanced by estimating a drift voltage per unit time output by an amplifier of a distance measuring circuit prior to a distance measurement operation and subtracting the influence of drift voltage from a result of the distance measurement after the distance measurement operation has been performed.
However, in the above-described distance measuring device, the influence of drift voltage is removed in terms of software, and detection of the drift voltage by software inevitably results in an error because it is different in sequence from the actual distance measuring operation. Thus, an improvement in accuracy of distance measurement cannot be accurately obtained. Also, since the drift voltage is added to an output of a pair of integrating circuits, such output will be made different from a designed value. In the case of using such output, there is also caused a problem of yield rate because the integrating circuits are substantially changed in dynamic range. Also, the drift voltages superimposed on an output of the integrating circuits include not only the drift voltage of an amplifier different from the integrating circuits but also drift voltages of the integrating circuits themselves. Conventionally, both such drift voltages have not been removed in terms of circuitry.
To solve the above-mentioned problems, the present invention provides a distance measuring device comprising light projecting means for irradiating light onto an object undergoing distance measurement, light receiving means for receiving light, reflected by the object and generating two outputs varying in value depending upon a position of received light, an amplifier for amplifying the two outputs on the basis of a reference voltage, a regulating circuit for regulating the reference voltage, an integrating circuit for integrating two outputs of the amplifier separately, and calculating means for finding a distance to the object making use of the two outputs thus integrated, wherein the regulating circuit regulates the reference voltage on the basis of offset voltages the amplifier and the integrating circuit. Accordingly, offset voltages of the amplifier and the integrating circuit can be canceled on the basis of an output of the integrating circuit to achieve enhancement of accuracy in distance measurement. Also, since offset voltages of the amplifier and the integrating circuit can be canceled on the basis of an output of the integrating circuit, an output of the integrating circuit becomes stable and an improvement in yield is attained.
Also, the device preferably further comprises a storage circuit for storing a value conformed to offset voltages of the amplifier and the integrating circuit, and the regulating circuit preferably regulates the reference voltage on the basis of offset voltages of the amplifier and the integrating circuit stored in the storage circuit, so that the device can be made simple in operation because detection of a value conformed to offset voltages of the amplifier and the integrating circuit every time is dispensed with by storing at one time the value conformed to offset voltages of the amplifier and the integrating circuit in the storage circuit and regulating a reference voltage in accordance with the stored value.
The regulating circuit may comprise an analog voltage output circuit, regulating resistors and a switching element, and use the analog voltage output circuit and the regulating resistors to generate a regulating voltage to output the same to the amplifier via the switching element.
Further, offset voltages of the amplifier and the integrating circuit may be based on an output of the integrating circuit when the light projecting means does not operate.
Prior to distance measurement, the regulating circuit regulates a reference voltage on the basis of offset voltages of the amplifier and the integrating circuit, whereby offset voltages of the amplifier and the integrating circuit, which are contained in an output of the integrating circuit, can be cancelled, so that it becomes possible to enhance accuracy in distance measurement performed by the calculating means.